The present invention relates to a process for the production of polymers, preferably water-absorbing polymer structures, by radical polymerization of acrylic acid, the water-absorbing polymer structures obtainable by this process, water-absorbing polymer structures which are based to at least about 25 wt. % upon partially neutralized acrylic acid, a composite, a process for the production of a composite, the composite obtainable by this process, the use of acrylic acid in the production of polymers, preferably in the production of water-absorbing polymer structures, a device for the production of acrylic acid, a process for the production of acrylic acid, and the acrylic acid obtainable by this process.
High demands are made of the purity of monomers which are used in the production of polymeric compounds. This is particularly the case if the polymers are so-called superabsorbent polymers. These polymers are capable of absorbing and thereby binding aqueous liquids to form a hydrogel. Superabsorbent polymers are, therefore, used in particular in hygiene articles such as diapers, incontinence articles, sanitary napkins, and the like for the absorption of body fluids. A comprehensive overview of superabsorbent polymers, their application, and their production is given by F. L. Buchholz and A. T. Graham (Editors) in “Modern Superabsorbent Polymer Technology”, Wiley-VCH, N.Y., 1998.
Superabsorbent polymers may be produced from pure acrylic acid prepared by catalytic gas phase oxidation of propylene to acrolein, wherein the acrolein is then converted in a further catalytic gas phase oxidation to acrylic acid. Then the acrylic acid is further processed by the gaseous reaction mixture in water, distillation of the thus-obtained aqueous acrylic acid solution to obtain a crude acrylic acid, and further purification of the crude acrylic acid by means of distillation or crystallization.
It is a disadvantage of this process for production of acrylic acid that the temperatures between about 300° C. and about 450° C. applied in both steps lead to formation of oligomers and further undesired cracking products. This has the result that an undesirably large amount of compounds which are less volatile than acrylic acid, or compounds which are only separated from acrylic acid with difficulty, such as, for example, acetic acid, is formed. These compounds must generally be separated from the acrylic acid by distillation, which in turn leads to a further thermal stress on the acrylic acid and to the formation of dimers and oligomers which is linked therewith. A high content in acrylic acid dimers or acrylic acid oligomers is, however, disadvantageous, since these dimers or oligomers are incorporated into the polymer backbone during the production of superabsorbent polymers by radical polymerization of acrylic acid in the presence of crosslinkers. During the post-treatment of the surface of the polymer particles occurring after the polymerization, for example during a surface post-crosslinking, the polymerized-in dimers, however, are cleaved to form β-hydroxypropionic acid, which is dehydrated to form acrylic acid under the post-crosslinking conditions. A high content in dimeric acrylic acid in the acrylic acid used in the production of superabsorbent polymer therefore leads to increased content in acrylic acid monomers upon thermal treatment of the polymer, as occurs during the post-crosslinking.
Since the soluble parts, in particular the acrylic acid monomers, in superabsorbent polymers can cause skin irritation, the use of these polymers in hygiene articles requires a particularly low content in extractable components.
Also other, often toxic compounds are still comprised in the acrylic acid obtainable by catalytic gas phase oxidation. These impurities include, in particular, aldehydes, which have a disruptive effect upon the course of polymerization, with the result that the polymers comprise considerable amounts of soluble components.
Acrylic acids produced in previous ways from propylene comprise not inconsiderable amounts of ketones having double bonds, in particular protoanemonine (PTA). This compound can, on contact with skin, cause signs of poisoning, such as, for example, reddening, itching, or blister formation. Superabsorbent polymers which comprise large amounts of PTA as soluble components are therefore of concern from a dermatological viewpoint. Furthermore, PTA disrupts the polymerization, as described in US-A-2002/0120085. This leads to the obtaining of superabsorbent polymers with less good absorption, transport, and retention properties for body fluids, so that when using superabsorbent polymers of this type in hygiene articles such as diapers or sanitary napkins, wearer comfort is worsened, for example by “leakage”.
Some processes have already been described in the state of the art, with which the content in the above-mentioned compounds, in particular of aldehydes or PTA in acrylic acid obtained by gas phase oxidation of propylene can be reduced.
DE-A-101 38 150 suggests, in order to reduce the amount of aldehyde in the acrylic acid, bringing this into contact with an aldehyde trapper, in order to convert the aldehydes into high-boiling compounds, which can then be separated by means of distillation.
Various methods have been proposed in the state of the art for the removal of PTA, such as the addition of a nitrous acid salt, of nitrogen oxide or of nitrobenzene (JP 81-41614) or the addition of one or more para-phenylene diamines (EP-A-567 207) to the acrylic acid.
The disadvantage of the above-described processes for reducing the amount of aldehydes and ketones in acrylic acid is, however, among others, that, in so far as the impurity content of the acrylic acid is not known exactly, these reagents must be used in excess for the purpose of as complete a removal as possible of impurities from the acrylic acid. On the one hand, reagents which are reactive to the acrylic acid must be added. The portion of these reagents which is not converted must then be removed again. Reagents which are not removed are comprised in the superabsorbent polymer obtained from such an acrylic acid as soluble components, which can come into contact with the skin of the hygiene article wearer when the superabsorbent polymers are used in hygiene articles. Furthermore, the processes known from the prior art for removal of aldehydes in ketones from acrylic acid only very seldom remove these impurities completely.
In addition to the disadvantages which are traced back to impurities in the acrylic acid used in the production of superabsorbent polymers, known superabsorbent polymers also have the disadvantage that, unless they at least partially comprise natural polymers, such as celluloses, they are hardly based upon renewable raw materials. While it is successful to produce many of the components used in hygiene articles, in particular in disposable diapers, from biological starting materials, replacement of the superabsorbent polymers based upon cross-linked polyacrylates by natural superabsorbent polymers, such as cross-linked, derivatized starches or celluloses, is generally associated with significant losses in respect of the absorbent properties. This mostly leads to the necessity of using considerably more of the absorbents based upon natural polymers, simply in order to approach the same absorbent properties in a hygiene article. This is disadvantageous, because the hygiene articles become more voluminous and heavier, which significantly restricts wearing comfort and leads to a larger waste volume, which, in addition to dumping space or combustion expenditure also requires greater transport capacity for the removal of waste. All of this has a disadvantageous effect upon the environmental friendliness of the absorbers based upon natural polymers.